As shown in FIG. 13, there is known a reclining device that includes an internal gear 15 and an external gear 17. For example, the internal gear 15 is provided in a seat back, and the external gear 17 is provided in a seat cushion. The number of teeth of the external gear 17 is less than that of the internal gear 15, and the external gear 17 is engaged with the internal gear 15. The reclining device is configured to eccentrically move one of the internal gear 15 and the external gear 17 around a rotation axis of the other gear and to change the engagement position of the internal gear and the external gear, thereby changing a tilt angle of the seat back relative to the seat cushion.
FIG. 13 is a view showing the internal gear 15 and the external gear 17. The internal gear 15 is a dish-shaped member. The internal gear 15 has a bottom surface, at the back side of the paper surface of FIG. 13, and a ring-shaped rib extending, toward the front from the paper surface, is provided in an outer peripheral edge of the bottom surface. Internal teeth 15a are provided on the rib. Further, a cylindrical part 3 extending toward the front from the paper surface is provided in the bottom surface of the internal gear 15.
The ring-shaped external gear 17 is provided between the internal teeth 15a of the internal gear 15 and the cylindrical part 3. External teeth 17a are provided on an outer peripheral surface of the external gear 17. A circular hole 1 is formed by an inner peripheral surface of the external gear 17.
As shown in FIG. 13, an eccentric annular space is formed between an inner surface of the circular hole 1 of the external gear 17 and an outer surface of the cylindrical part 3 of the internal teeth 15. A first wedge-shaped member 5 and a second wedge-shaped member 7 are arranged in the eccentric annular space. The first wedge-shaped member 5 and the second wedge-shaped member 7 are urged by a spring 9 in a direction (a direction of an arrow A and a direction of an arrow B) in which a wedge is driven into the eccentric annular space.
When the first wedge-shaped member 5 and the second wedge-shaped member 7 are pressed against the inner surface of the circular hole 1 and the outer surface of the cylinder part 3, the internal gear 15 and the external gear 17 are urged in a direction in which the amount of eccentricity between the rotation axes of both is increased. In this way, the internal teeth 15a of the internal gear 15 and the external teeth 17a of the external gear 17 are deeply engaged, so that so that the seat back is not tilted (locked state: non-operation state).
An unlocking cam 11 is configured such that an abutting surface 11a presses an end surface 5a or an end surface 7a on the wedge tip side of the first wedge-shaped member 5 and the second wedge-shaped member 7 against an urging force of the spring 9, and the first wedge-shaped member 5 or the second wedge-shaped member 7 are pressed in a direction in which the wedge-shaped members are pulled out (a direction opposite to the driving direction: a direction opposite to the direction of the arrow A or the arrow B). When the first wedge-shaped member 5 or the second wedge-shaped member 7 is moved by being pressed by the unlocking cam 11, a press-contact force of the first wedge-shaped member 5 or the second wedge-shaped member 7 to the inner surface of the circular hole 1 and the outer surface of the cylindrical part 3 is decreased. In this way, the engagement between the internal teeth and the external teeth becomes shallow, so that the seat back is in a tiltable state.
At the start of movement of one of the first wedge-shaped member 5 and the second wedge-shaped member 7, the other of the second wedge-shaped member 7 and the first wedge-shaped member 5 remains stationary by friction between the inner surface of the circular hole 1 and the outer surface of the cylindrical part 3. However, as the one of the wedge-shaped members moves in the pull-out direction, the other wedge-shaped member moves all at once by an elastic repulsive force of the spring 9 in a direction in which the wedge is driven into the eccentric annular space. As the above operation is repeated, an eccentric state between the internal gear and the external gear is maintained and the engagement location is changed, so that the seat back is tilted (unlocked state: operation state) (e.g., see Patent literature 1).
Now, in a seat using a conventional reclining device, a force applied to the reclining device will be described with reference to FIG. 14. In addition, in FIG. 14, the same parts as in FIG. 13 are denoted by the same reference numerals, and a duplicated description thereof is omitted. The reclining device shown in FIG. 14 is different from the device shown in FIG. 13 in that a drive ring 21 is provided. The drive ring 21 is movably provided between the inner surface of the circular hole 1, and the first wedge-shaped member 5 and the second wedge-shaped member 7. The drive ring 21 has an abutting portion which is abutted against the first wedge-shaped member 5 and the second wedge-shaped member 7. The first wedge-shaped member 5 and the second wedge-shaped member 7 are clamped by the drive ring 21 and the outer surface of the cylindrical part 3.
In the reclining device shown in FIG. 14, there is an assumed case where:
the internal gear 15 (cylindrical part 3) is provided in the seat back;
the external gear 17 (hole 1) is provided in the seat cushion;
an inclined state of the seat back corresponds to the state of being inclined in an angle of 21 degrees rearward from a vertical state;
a vertical downward load applied to the seat back due to its own weight is 5 kgf;
a vertical downward load applied to the seat back due to a seated person is 15 kgf; and
a horizontal distance between an engagement position of the internal gear 15 and the external gear 17 and a point of action of the load applied to the seat back is 89.2 mm.
In this case, a moment applied to the engagement position of the internal teeth 15a and the external teeth 17a is expressed as 89.2 mm×20 kgf=1784 kgf-mm.
In the conventional reclining device shown in FIG. 14, pressure angles of the internal teeth 15a and the external teeth 17a are set so as to prevent slippage between the teeth surfaces of the internal teeth 15a and the teeth surfaces of the external teeth 17a when a load from the seat back is applied to the engagement position. Therefore, the load (force) from the seat back, which is applied to the engagement position, is transmitted in this order in the path of the cylindrical part 3 of the internal gear 15, the first wedge-shaped member 5, the circular hole 1 of the external gear 17, and the seat cushion.
Since in a direction perpendicular to a force applied to the first wedge-shaped member 5 from the cylindrical part 3 of the internal gear 15, a distance between a point of action of the force and an engagement position of the internal gear 15 and the external gear 17 is 29.8 mm, a force F transmitted to the external gear 17 from the first wedge-shaped member 5 becomes F=1784/29.8=59.86 kgf
At this time, at the engagement position, a force (a component of a reaction force F shown in the figure) of causing the internal teeth 15a and the external teeth 17a to be slipped is expressed as follows.(A perpendicular force(S) to the teeth surfaces of a reaction force of F occurring at the engagement position×tan (7.8 degrees))=(the perpendicular force(S) to the teeth surfaces of the reaction force of F occurring at the engagement position×0.137).
Generally, a coefficient (μ) of static friction at the engagement position of the teeth and the teeth is 0.2. Therefore, the maximum static frictional force is equal to (the perpendicular force(S) to the teeth surfaces of the reaction force of F occurring at the engagement position×0.2).
At the engagement position, the force of causing the internal teeth 15a and the external teeth 17a to be slipped is less than the maximum static frictional force. Therefore, the slippage between the internal teeth 15a and the external teeth 17a at the engagement position does not occur.